In numerous aircraft, and in particular for improving drag and fuel consumption, undercarriages are provided that are mounted in retractable or foldable manner.
Amongst such undercarriages, the invention relates more particularly to “straight” landing gear.
In order to be retractable, straight landing gear comprises a leg with at least one wheel mounted at its bottom end, the leg being pivotally mounted about a transverse main axis in order to be capable of being retracted into a well in the fuselage, under drive from a retraction actuator.
At present, anti-crash functions are rare in retractable straight landing gear. When such functions are present they are in practice limited or inadequate according to the applicable standards.
By way of illustration, mention is made of patent document FR 2 687 123 which describes a retractable helicopter landing gear.
In order to deal with a crash landing situation in spite of the short stroke of its resilient leg, that landing gear includes an additional device for absorbing energy and presenting a force threshold, which device is integrated in the driving actuator.
In the event of a crash landing, the actuator remains hinged to the landing gear leg and the absorption device allows the landing gear to rise with controlled pivoting.
With such straight landing gear, two main stages are observed during a crash, giving rise to anti-crash functions that are dedicated, i.e. that are specific for each stage, and in particular:
a first stage when the undercarriage comes into contact with the landing surface, during which it is appropriate to provide a first damping function seeking to absorb the energy of axially shortening the undercarriage; and
a second stage of a crash, generally after the first stage, during which it is appropriate to brake pivoting of the landing gear leg.
In practice, proposals that have been made in the past for retractable straight undercarriages have related solely to anti-crash functions that are specific to the first stage.
In such undercarriages, this specific function is damping that is obtained by means of a sliding tube (e.g. made of composite material, and in particular of carbon).
In the event of a crash, the tube can slide substantially along the direction of the central axial fiber of the landing gear leg, and relative to the axis about which the leg is retracted into the well. In normal, non-crash operation, this sliding is prevented by a shear bolt.
If the speed with which the wheels make contact on landing reaches a threshold value equal to a crash speed, this causes the shear bolt to shear.
This shearing in turn releases the sliding tube so that it can slide away from the functional position of the landing gear leg.
The tube then moves substantially along the direction of the central axial fiber of the landing gear leg at substantially constant force, absorbing the energy generated during the first stage of a crash.
During the first stage of a crash, the normal retraction function of the actuator is inhibited because of the axial shortening speeds of the undercarriage (of the order of 8 meters per second (m/s) to 11 m/s, as compared with less than 1 m/s during a normal landing).
According to document FR 2 687 123, once the retraction function has been inhibited, the actuator absorbs the energy coming from the pivoting of the landing gear leg about its main retraction axis.
It is this absorption device integrated in the control actuator that limits and controls pivoting, which constitutes the main part of the second stage of a crash.
However with known straight landing gear, the landing gear leg tends to rise violently towards the inside of the aircraft, beyond the well for receiving it.
In addition, the inhibition of the retraction actuator at the end of the first stage of a crash allows the compression energy of the undercarriage tires to be released.
This energy release further increases the upward pivoting acceleration of the landing gear leg during the second stage of a crash.
Furthermore, with certain existing straight landing gear, during a crash, there is a longitudinal offset in the forward direction of the aircraft between the central axial fiber of the leg and the axis of its wheel(s)
Such an offset is common, e.g. in steerable nose landing gear, in order to make the aircraft easier to taxi by steering its wheels.
Because of the additional lever arm this generates, it is found during the second stage of a crash that acceleration of the landing gear leg as it rises is increased.
This leads to initial pivoting of the undercarriage during the second stage of a crash, typically through an angle of about 90° with an angular speed of substantially 40 radians per second (rad/s), for example.
For a retractable straight undercarriage weighing about 70 kilograms (kg), the force generated in this way can be of the order of 35000 newtons (N).
There is then a risk of structural damage being generated in particular to the top wall of the landing gear well and the adjacent structures, e.g. within the floor of the cabin for nose landing gear.
Unacceptable human injury is also likely to be caused during the second stage of a crash if the people are close to the landing gear.
At this stage, mention can be made of other documents in the technical field of anti-crash installations for aircraft.
Document FR 2 472 115 describes equipment for braking a body in movement, the equipment comprising strips that are theoretically not elastic and that are suitable for receiving the impact of the moving body.
The strips are connected to adjustable braking means.
Document FR 2 316 483 describes an auxiliary device for absorbing kinetic energy for an airplane stop barrier situated on a landing ground or zone.
That device constitutes a brake made entirely of textile material, having no mechanical moving parts, and of small bulk. The brake comprises coiled straps held by destroyable retaining means such as stitching and connected to the barrier.
As a result, when an airplane for stopping is caught in the barrier, the straps separate progressively by breaking fibers, thereby braking and then stopping the airplane.
Applications for such equipment are to be found in particular at the ends of runways or on aircraft-carrier landing decks. Furthermore, the equipment may be associated with a load for releasing from an airplane, helicopter, or balloon, in order to protect it from the effects of making contact with the ground.
Document FR 2 684 957 describes a peak-limiter device for shock absorbers for helicopter landing gear.
A shock absorber comprises a piston rod disposed at the bottom and a main cylinder at the top. The piston rod and the main cylinder are hinged via end ball-joints, respectively at the bottom to a component of the landing gear and at the top to a structure of the helicopter.
That landing gear is not steerable, and the peak-limiter device is always integrated in the shock absorber.
Mention is also made of documents of interest relating to landing gear.
Document FR 2 689 087 describes a retractable undercarriage for an aerodyne, such as a helicopter.
A landing gear leg is pivotally mounted on a structure of the aerodyne to pivot about an axis that is essentially perpendicular to its longitudinal midplane.
A controlling actuator is associated with the landing gear leg to extend it and to retract it.
In addition, the actuator serves as a force threshold shock absorber, applying a predetermined torque tending to hold the landing gear leg in the position it occupies during landing, while allowing it to rise under controlled force in the event of a crash landing.
Above a predetermined threshold, corresponding to a crash landing situation, the wheels of the undercarriage transmit a large force to the undercarriage leg.
When the force threshold is exceeded, the shock absorbing actuator allows the landing gear leg to pivot in the retraction direction.
Document FR 2 689 088 describes a shock absorbing actuator for a helicopter that includes a function of limiting force in the event of a crash.
It also includes a brace and performs functions of controlling the undercarriage, absorbing shock, and peak-limiting forces.
U.S. Pat. No. 5,944,283 describes a shock absorber for an anti-crash undercarriage. Its rocker axis is offset.
However the teaching of the above, in particular, does not make it possible to obtain anti-crash functions that are effective for a retractable straight airplane undercarriage.
In particular, functions are not available in practice for providing structures, and humans, on board the aircraft with effective protection during the second stage of a crash.
In order to make this second stage of a crash safer, adapting solutions that are designed for the first stage leads to numerous drawbacks that are excessive if not insuperable.
In providing protection during this second stage of a crash, it is also possible to comply in particular with the following criteria:                major modifications to the environment of an undercarriage (preexisting or being designed) should be avoided;        the additional on-board weight should be small or negligible:        the additional bulk should be small or negligible;        safety should be provided effectively against the forces involved;        the dynamics involved should be compatible with normal operation and first stage anti-crash operation;        cost and ease of installation and maintenance should be as small as possible; and        the additional means should be both robust and long-lasting.        
These criteria are even more severe for rotary wing aircraft where questions concerning undercarriage size and stability during landing are particularly constraining.
In terms of safety during the second stage of a crash, the invention ought to provide, for example:                compatibility with an impact speed of about 7 m/s to 9 m/s, e.g. about 8.2 m/s;        compatibility with the undercarriage pivoting during the second stage at a speed greater than or equal to 10 rad/s;        the landing gear leg should be allowed to pivot freely and in controlled manner before anti-crash braking is applied, over respective predetermined angles; and        the maximum impact forces on the structure of the aircraft (e.g. the floor above the landing gear well for a nose undercarriage) at the end of the second stage stroke of the undercarriage compatible with the strength of the surrounding structure, in order to avoid damaging it.        
For example, up to a value of about 25,000 N, crash impact forces should not be transmitted to the take-up structure, and the energy thereof should be absorbed.
It is also appropriate that safety standards, both present standards and future standards, should be complied with in order to ensure that an aircraft of the invention can obtain type approval quickly and easily.